U-Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P-T-t path

Abstract

Laser ablation inductively coupled plasma mass spectrometry analyses of U-Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh-pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction-zone metamorphism. Zircon U-Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U-Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti-in-zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8°C and 742 ± 19 to 778 ± 34°C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction-zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti-in-zircon temperatures for the younger zircon at lower pressures indicate the initial ÔhotÕ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE-HREE patterns and no Eu anomalies, and yields Zr-in-titanite temperatures of 551 to 605°C at 0.5 GPa, and the other shows LREE depletion and flat MREE-HREE patterns, and gives Zr-in-titanite temperatures of 782-788°C at 2.0 GPa. The former is amenable for U-Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ÔhotÕ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction-zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P-T-t path of eclogites during the continental collision. It appears that the mid-T ⁄ UHP eclogite facies zone would not only form by subduction of the continental crust in a P-T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.

Dehydration melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet

“…, 1995; Carswell et al. , 1997) and Ti‐in‐zircon and Zr‐in‐titanite thermometries (Gao et al. , 2011).…”

Combined oxygen-isotope and U-Pb zoning studies of titanite: New criteria for age preservation

“…Titanite is more reactive than zircon, and it commonly recrystallizes. Thus, U-Pb titanite dates are often interpreted as metamorphic ages, recording the time of pulsed magmatic heating, fluid infiltration, or deformation (e.g., Verts et al, 1996;Storey et al, 2007;Shang et al, 2010;Gao et al, 2011). Pb diffusion is also a well-documented phenomenon in titanite, and many titanite dates are interpreted to indicate the time of cooling of the host rock below a certain temperature (e.g., Mezger et al, 1991a;Flowers et al, 2006;Warren et al, 2012) -cited as being anywhere between~500-800°C for typical geologic cooling rates (Mezger et al, 1991a;Cherniak, 1993;Scott and St-Onge, 1995;Verts et al, 1996;Kohn and Corrie, 2011;Spencer et al, 2013).…”

Cathodoluminescence guided zircon Hf isotope depth profiling: Mobilization of the Lu–Hf system during (U)HP rock exhumation in the Woodlark Rift, Papua New Guinea

“…Zircon has become one of the more widely utilized minerals in the characterization of (U)HP rocks where the goal is to interpret geochemical and isotopic data collected at the μm-scale in relation to the P-T-t evolution of the host rock. This is because zircon U-Pb ages can record the timing of processes associated with (U)HP metamorphism (Gao et al, 2011;Liati, 2005;McClelland and Lapen, 2013;McClelland et al, 2009;Rubatto et al, 1998) and because its composition can be related to host rock bulk mineralogy (Hanchar and VanWestrenen, 2007;Harley et al, 2007;Monteleone et al, 2007;Rubatto, 2002;Rubatto and Hermann, 2007;Scherer et al, 2007). This paper presents the results of an investigation into the behavior of the Hf isotope system in zircon formed during the exhumation of two quartzofeldspathic (U)HP host gneisses from the active Woodlark Rift (Fig.…”